There are many types of equipment and apparatus which utilize front surface mirrors in optical systems such as copying machines, microfilm printers and the like. Such front surface mirrors must have a high degree of flatness in order to perform their functions properly. Ground and polished plate glass has heretofore been used for this type of front surface mirrors. However, such glass is becoming obsolete and is becoming difficul to obtain. In many applications, this plate glass is being replaced by float glass which, however, does not have nearly as predictable flatness qualities as plate glass. It has been found because of this fact the yield of acceptable front surface mirrors made from float glass is very low, resulting in a waste of time and materials.
Flatness has traditionally been measured interferometrically and flatness requirements were normally expressed in interferometry terms, i.e., so many fringes over such an area. However, such an interferometry measurement has had a number of shortcomings. It is slow. By way of example, ten measurement per minute would be near the maximum for nearly any type of interferometry equipment heretofore available. Interferometry measurements require very accurate alignment. Generally, the test part in the measuring head must be aligned within two arc seconds before a clear (well defined) fringe pattern can be observed. Interferometry requires freedom from vibration because vibrations tend to obliterate the fringe patterns. Interferometry is not readily adaptable to measurement on large sheets of material. It is difficult to separate the fringe patterns produced by the back side when interferometry is performed on uncoated glass. Interferometry measurements require interpretation of fringe patterns which requires a skilled operator. It, therefore, can be seen that there is a great need for a different type of flatness testing apparatus which can overcome these limitations of interferometry.